Cathode-ray device which incorporates high-speed phosphors



Oct. 13, 1970 w, LEHMANN 3,534,211

CATHODE-RAY DEVICE WHICH INCORPORA'IES HIGH-SPEED PHOSPHORS Filed 001--5, 1967 WITNESSES INVENTOR 071 WM WiHi Lehmonn W W M ATTORNEY UnitedStates Patent O 3,534,211 CATHODE-RAY DEVICE WHICH INCORPORATESHIGH-SPEED PHOSPHORS Willi Lehmann, Murrysville, Pa., assiguor toWestinghouse Electric Corporation, Pittsburgh, Pa., a corporation ofPennsylvania Filed Oct. 3, 1967, Ser. No. 672,493 Int. Cl. C09k 1/10;H013 29/20 US. Cl. 31392 4 Claims ABSTRACT OF THE DISCLOSURECROSS-REFERENCES TO RELATED APPLICATIONS In co-pending application Ser.No. 672,494, filed concurrently herewith by the present applicant isdisclosed a high-speed phosphor and method, as well as a cathoderaydevice, wherein the phosphor is cadmium sulfide or modified cadmiumsulfide doped with aluminum, gallium or indium. While the matrixmaterial of this cadmium sulfide phosphor is different from the presentphosphor, the method of preparation is quite similar.

BACKGROUND OF THE INVENTION This invention broadly relates to high-speedphosphors and, more particularly, to cathode-ray devices whichincorporate a particular high-speed phosphor, the high speed phosphorcomposition, and the method for preparing same.

Phosphors which have a very rapid decay time are known as high-speedphosphors. Such materials have use in flying-spot scanners, controlphosphors in color television tubes, scintillation counters, andexperimental or manufacturing control phosphors such as might be used todetermine a transit time and spread for photomultiplier tubes, to name afew of the applications.

Zinc oxide phosphor materials are well known in the art and respond toexcitation by ultraviolet or cathode rays, for example, to emitprimarily in two emission hands, a broad band in the green region of thevisible spectrum, and the so-called edge-emission which for thisphosphor is relatively narrow band in the near ultraviolet with a peakat about 390 nm. The green emission for such phosphors is normallyobtained by firing Zinc oxide under slightly reducing conditions. Theso-called edge-emission of this material has been reported to beobtained by firing pure zinc oxide under oxidizing conditions, seeLeverenz, An Introduction to Luminesce of Solids, published by Wiley,1950, page 218. The green emission of the Zinc oxide which is firedunder the slightly reducing conditions is obtained under excitation withultraviolet or cathode rays and the decay time is usually in the orderof 1X10 second. The foregoing ultraviolet edge-emission of pure zincoxide which is fired under oxidizing conditions is not readilyobtainable at room temperature unless the phosphor is excited by veryhigh energy cathode rays and, even then, the response of the phosphor iscomparatively weak. The decay time of this phosphor is reported to beabout 2X10 second by D. Hahn and K. Lertes in Z. Phys, 170, 367 (1962)and about 10- second by Leverenz at page 294 in Luminescence of Solids,Wiley and Sons, New York (1950).

In explanation of the term edge emission, this can be defined as aphosphor emission which occurs at a wavelength which is only slightlylonger than the long wavelength edge of the phosphor absorptionspectrum.

The decay time is the primary factor in determining the so-called speedof the phosphor, since the speed of initial response of the phosphor toexcitation is at least as fast as the speed at which the emission decaysafter excitation is removed, and is normally faster. The phosphor decaytime as measured herein is that period of time required for the phosphorbrightness to decay from a maximum value to a value which is 37% of themaximum value.

The term dope or dopant as used herein refers to a donor impurity whichconstitutes a lattice defect that is able to supply at least one freeelectron into the conduction band, or alternatively, an impurity thatenhances the n-type conductivity of the material.

SUMMARY OF THE INVENTION It is the general object of the presentinvention to provide a phosphor material which has an extremely highspeed.

-It is a further object to provide a cathode-ray device incorporating aphosphor which has an extremely high speed.

It is another object to provide a method for preparing a phosphormaterial which has an extremely high speed.

The foregoing objects of the invention, and other objects which willbecome apparent as the description proceeds, are achieved by providing aphosphor composition having a matrix of zinc oxide or Zinc-cadmium oxidewherein the gram-atom ratio of zinc to cadmium is at least 9:1. Thismatrix is doped with predetermined amounts of aluminum, gallium orindium. Due to the conditions of preparation, the electricalconductivity of the phosphor is substantially at a maximum possiblevalue, because of the substantial elimination of all traps in thephosphor. There is also provided a cathode-ray device which incorporatessuch a phosphor as Well as a method for preparing the phosphor.

BRIEF [DESCRIPTION OF THE DRAWING For a better understanding of theinvention, reference should be made to the accompanying drawing whereinthe sole figure is a plan view, partly in section, of the cathode-raydevice incorporating a phosphor screen which comprises the veryhigh-speed phosphor of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS To prepare the present phosphorthere is mixed together Group II-B metal compound of zinc or zinc pluscadmium wherein the gram-atom ratio of zinc to cadmium in the compoundis at least 9:1, and Group IIIA metal compound of aluminum, gallium,indium or mixtures thereof in such amount that the ratio of totalgramatoms of Group IIIA metal to Group IIB metal present in the mixtureis from 0.0001 to 0.05. The compound of the foregoing Group IIB metal isthe oxide or a compound which readily decomposes to the oxide onheating, and the compound of Group IIIA metal is the oxide or a compoundwhich readily decomposes to the oxide on heating.

The foregoing mixed compounds are fired in an oxidizing atmosphere at atemperature of from about 800 C. to about 1000 C. for at least about onehalf hour.

Thereafter, the first fired material is refired either in a hydrogenatmosphere or in an atmosphere of zinc vapor at a temperature of fromabout 550 C. to about 700 C. for a sufficient period of time to causethe resulting fired material to display substantially that maximumpossible value of electrical conductivity as is permitted by the amountof the aluminum, gallium, or indium in the phosphor.

EXAMPLE I As the first example, 3.75 grams of A1(NO -9H O is dissolvedin 100 ml. of water, which provide an aluminum concentration of 10*gram-atomic weight per ml. Thirty ml. of this solution are added to 81.4grams (1 gram mole) of pure zinc oxide powder and the mixture thoroughlyagitated as a slurry, and then dried. The dried powder is placed in aquartz container and fired in air at a temperature of from about 800 C.to 1000 C. for approximately one hour. The resulting first-firedmaterial at best is very poorly luminescent. This first-fired materialis refired in flowing hydrogen gas at a temperature of 600 C. to 700 C.for approximately 30 minutes. After cooling, the resulting phosphordisplays an edge-emission having a peak at about 390 mm. when excited by365 nm. ultraviolet, or by cathode rays. The phosphor has a speed ofresponse, i.e., a decay time, which is so rapid that it could not bemeasured by available apparatus, which was limited to measuring a decaytime of no less than about 5 10 second. It is estimated that the decaytime of the phosphor is in the order of about 10 second.

EXAMPLE II 159 milligrams of dry aluminum oxide powder are mixed with81.4 grams of dry zinc oxide powder by means of ball milling.Thereafter, the raw mix is prefired and then fired in flowing hydrogengas in the manner as described in Example I. The resulting phosphor isidentical to that achieved under Example I.

EXAMPLE III 700 milligrams of gallium metal are dissolved in hot nitricacid, the excess acid is boiled off, and the resulting solution thendiluted to 100 ml. Thirty ml. of this solution (which corresponds to 10"gram atomic weight of gallium per ml.) are added to 81.4 grams of zincoxide powder and the raw mix constitutents are formed into a uniformslurry and then dried. Thereafter, the firing procedure is that which isdescribed in foregoing Example I and the luminescence of the resultingphosphor is substantially identical to that reported under Example I.

EXAMPLE IV Aluminum is added either as a dry oxide or as a compound insolution, as per the foregoing examples, to pure Zinc oxide. The mixtureis prefired in accordance with the foreging examples. The prefiredmaterial is then enclosed in a quartz tube to which is addedapproximately 100 to 200 milligrams of zinc metal. The tube is evacuatedand sealed off. The entire tube is then fired at a temperature ofapproximately 700 C. for approximately minutes so that the actual firingatmosphere for the phosphor raw mix constitutes an atmosphere of zincvapor. After cooling, the phosphor is substantially identical to thatreported under Example I.

In the foreging examples,, the concentrations of aluminum, gallium,indium, or any mixtures thereof, in compound form as specified, may varybetween about 0.0001 to 0.05 gram atom per gram atom of total zinc pluscadmium in the phosphor matrix. The preferred dopant is aluminum orgallium or mixtures thereof present in amount of about 0.003 gram atomper gram atom of total zinc in the phosphor matrix.

While the foreging examples all relate to finely divided or powderphosphor, the present phosphor can also be formed as a thin film. As anexample, metallic zinc or Zinc telluride is evaporated as a thin filmonto a substrate. The evaporated and deposited zinc or zinc telluride isthen converted to the oxide by heating in air. The dopant is preferablyco-deposited as a metal with the matrix compound and then converted tothe oxide. After conversion to the oxide, the film is heated in thehydrogen or zinc vapor atmosphere in the manner as describedhereinbefore.

The final firing procedure in the hydrogen atmosphere or the atmosphereof zinc vapor, at atemperature of from about 550 C. to about 700 C.should be conducted for a sufficient period of time to cause theresulting fired material to display substantially that maximum possiblevalue of electrical conductivity as is permitted by the amount of thedopant in the matrix. In explanation of this conductivity, the finalfiring under the specified conditions causes the total zinc plus cadmiumin the matrix to exceed that amount which is required to form thestoichiometric oxide, and apparently this serves to fill all traps onthe matrix, thereby accounting for the extremely fast decay since thetraps account for the phosphorescence or after-glow. As a result,substantially each atom of the dopant in the phosphor contributes onefree electron to the electrical conductivity of the phosphor. This isbest illustrated by the results reported in the following Table I:

TABLE I {Influence of hydrogen firing atmosphere on electrical powderresistance 1 and on color or emission of photoluminescence at roomtemperature] No dopant added 0.3% Ga added Ohms Color Ohms Emission 2,500 Dead.

120, 000 Dead 300, 000 Weak green.

No refiring 1 Measured with 1.5 volts D .C. applied to a powderlayer of1 cm. area and 1 mm. thickness pressed between two brass electrodes.

The foregoing measurements are of a qualitative nature since themeasurement of electrical resistance of a bulk powder is not veryreliable. The foregoing results clearly indicate, however, the vastdifference in electrical conductivity which is obtained using thehydrogen firing atmosphere in contrast to the elimination of thehydrogen refiring. The measured conductivity will vary with the amountof dopant which is included within the specified ranges. To illustrate,the calculated value of conductivity for non-particulate (i.e., solid)zinc oxide doped with the minimum specified amount of 0.0001 gram-atomper gram-atom of zinc is about 2 mhos, and the calculated value ofconductivity for the maximum dopant amount of 0.05 gram-atom is about 10mhos.

In the foregoing examples, the zinc can be replaced by cadmium oxide insuch amount that the gram-atom ratio of zinc to cadmium is at least 9:1.The zinc oxide in the raw mix could be replaced by a compound whichreadily decomposes to the oxide on heating, such as the nitrate. Also,the doping compounds can be mixed in any relative proportions in theforegoing examples.

With specific reference to the form of the invention illustrated in thesole figure of the drawing, the numeral 10 illustrates a cathode-raydevice which is generally of conventional construction and comprises anevacuated envelop 12 including a faceplate 14. An electron gun 16 ispositioned within the neck portion of the envelope 12 and the device isprovided with a conventional electron deflecting means, illustrateddiagrammatically as defleeting coils 18 and 20. A phosphor screen '22 ispositioned between the faceplate 14 and the electron gun 16 and thisphosphor screen comprises the high-speed phosphor of the presentinvention. It should be clear that the tube 10 as illustrated is onlyshown in diagrammatic form and that the present high-speed phosphor canbe used equally well in a color television tube as a control phosphor,in which case the present phosphor would be.

5 used in conjunction with conventional color T.V. tube phosphors,shadow mask, plural guns, etc.

The present phosphor also has application in fiying spot scanners andscintillation counters and because of the extremely fast decay time,there exist many other uses for this phosphor.

It will be recognized that the objects of the invention have beenachieved by providing a phosphor composition which has an extremely fastrate of decay as well as a method for making such a phosphor. There isalso provided a cathode-ray device which incorporates such a high-speedphosphor.

I claim as my invention:

1. A cathode-ray device comprising:

(a) an evacuated envelope including a faceplate;

(b) electron-gun means spaced from said faceplate within said envelope;and

(c) a phosphor screen positioned between said faceplate and saidelectron-gun means, and said phosphor screen comprising a high-speedphosphor composition which emits primarily in the near ultraviolet andhas a matrix of zinc oxide or zinc-cadmium oxide wherein the gram-atomratio of zinc to cadmium is at least 9:1, said matrix is doped withaluminum, gallium, indium or mixtures thereof in amount of from 0.0001to 0.05 gram-atom per gram-atom of total zinc plus cadmium in saidmatrix, and the electrical conductivity of said phosphor beingsubstantially at that maximum possible value as is permitted by theamount of said dope in said phosphor.

2. The cathode-ray device as specified in claim 1, wherein the totalzinc plus cadmium in said matrix exceeds that amount required to formstoichiometric oxide.

3. The cathode-ray device as specified in claim 1, wherein substantiallyeach atom of said dopant in said phosphor composition contributes onefree electron to the electrical conductivity of said phosphor.

4. The cathode-ray device as specified in claim 1, wherein said dopantis aluminum or gallium or mixtures thereof present in amount of about0.003 gram-atom per gram-atom of total zinc plus cadmium.

References Cited UNITED STATES PATENTS 2,529,711 11/1950 Smith 252-30162,887,632 5/1959 Dalton 252301.6 3,089,850 5/1963 Cyr et a1. 2525013,130,341 4/1964 Johnson 31392 HELEN M. MCCARTHY, Primary Examiner R. D.EDMONDS, Assistant Examiner US. Cl. R.X. 252301.6

